This invention relates to extruders of the type in which a screw rotatable within a barrel is employed to extrude material to a die or injection mold connected to the outlet end of the barrel. The invention is concerned particularly with improvements in high output plasticating extruders.
A plasticating extruder receives polymer pellets or powder (often together with formulation additives in liquid or particle form), works and raises the temperature of the polymer sufficiently to dispose it in a melted or plastic state, and delivers the melted polymer under pressure through a restricted outlet or die. Ordinarily it is desirable that the discharge extrudate be fully melted, well mixed, uniform in temperature and pressure, and substantially free of small gels and other fine structure agglomerations. It also is desirable that the rate of delivery of the molten polymer through the die be regulatable simply by changing the rate of extruder screw rotation and that the rate of delivery at the selected screw speed be substantially uniform.
The basic extruder apparatus includes an elongated barrel which may be heated or cooled at various locations along its length and a screw which extends longitudinally through the barrel. The screw has a helical conveying land on its surface which cooperates with the cylindrical internal surface of the barrel to define an elongated helical channel.
An extruder screw ordinarily has a plurality of sections which are of configurations specifically suited to the attainment of particular functions. Examples are "feed" sections and "metering" sections, which are of basic importance and are present in nearly all plasticating extruders for handling thermoplastic polymers.
A typical extruder screw feed section extends beneath and forwardly from a feed opening where polymer in pellet or powder form is introduced into the extruder to be carried forward along the inside of the barrel by the feed section of the screw.
As the material is advanced along the channel, it is worked. This, in turn, generates heat, and melting of the polymer proceeds as the material is moved along the feed section and later sections of the screw. Actually, most of the melting occurs near the barrel surface at the interface between a thin melt film and the solid bed of polymer. This general pattern persists until a substantial portion of the polymer reaches the melted state. It is usually advantageous to employ a tapered transition section between the relatively deep feed section and the shallower metering section. Prior to solid bed breakup this keeps the solid bed width larger and more tightly pressed against the barrel wall, thereby enhancing the melting rate. After some 40% to 70% of the polymer has been melted, solid bed breakup usually occurs, and at this time particles of solid polymer become dispersed in the polymer melt.
An extruder screw "metering" section has as its special function the exertion of a pumping action on the molten polymer. Ordinarily, the throughput achieved by a screw is thought of as being a function of the combination of the "drag flow" and "pressure flow" effects of the metering section.
Drag flow is basically the flow which results from the relative movement between the screw and the internal surface of the extruder barrel. It may be thought of as being proportional to the product of the average relative velocity and the channel cross-sectional area. Stated in another way, the drag flow is the volumetric pumping capacity, the latter being a function only of screw channel dimensions times screw rpm. This drag flow component is directed toward the outlet end of the screw. It may be increased by increasing the speed of the screw and/or by increasing the depth of the flow channel in the screw.
Acting in opposition to drag flow is a pressure flow component stemming from the reluctance of the material to flow through the restricted outlet opening at the end of the extruder passage. The speed of the screw does not directly affect the pressure flow component but of course it may affect such factors as back pressure and material viscosity, which factors, in turn, affect significantly the pressure flow component. On the other hand, pressure flow is directly effected by both the depth and length of the screw channel; and increase in channel depth has a tendency to increase greatly the pressure flow component and an increase in channel length has a tendency to reduce this back flow component.
In the above-referenced patents of the present inventor there is described an extruder screw whose metering section includes one or more channels following a wave-like cylical pattern wherein each channel includes periodic wave peaks. The wave portion of the screw performs both metering and mixing functions. Insofar as metering or pumping is concerned, the systematically repeating wave pattern functions like conventional long metering sections of constant depth in the sense of providing uniform output approximately proportioned to screw rotational speed and providing normal resistance to pressure flow in a rearward direction along the screw channel.
In addition to its good metering properties, the wave portion of the screw has the advantage of achieving good mixing of the polymer without generating excessive heat. In the regions of the wave crests, the material is subject to high shear forces so that incompletely melted polymer will be worked and mixed vigorously with the molten material. The material passes from each zone of high shearing action into an adjacent zone of increased channel depth where the heat generating effects are much less intense.
This wave screw design has performed effectively to produce high quality melt. Notwithstanding this successful performance, efforts are continuously directed toward increasing the rate of extrudate production. In this regard it should be noted that in single channel extruders the maximum height of the wave crests is limited, and the wave crests may not begin to occur ahead of a location along the screw where considerable melting has already taken place. Otherwise, the presence of solid at the wave crests can create a serious restriction to extrudate flow.
It has been heretofore proposed in U.S. Pat. No. 3,701,512 issued Oct. 31, 1972 to Schippers et al in an effort to intensify melting and mixing of extrudate, to divide a single screw channel into a pair of side-by-side helical channels by an intermediate shearing thread. The channel depths would vary continuously and oppositely along the length of the passages (i.e., mirror images of one another) so that the combined passage cross-sectional area is maintained constant along the screw length. As one channel becomes diminished in depth, the adjacent channel depth becomes correspondingly enlarged, so that extrudate is expelled from the diminishing channel, across the shearing thread, and into the enlarging channel. Proposals such as this do not effectively deal with problems such as overheating and non-uniform temperatures of extrudate at the screw outlet, however. Moreover, these proposals do not optimize melting within a channel by solids enrichment and shear working at the wave peaks.
It is, therefore, an object of the present invention to provide methods and apparatus which minimize or obviate problems of the above-described sort.
It is another object of the present invention to provide novel extruding methods and apparatus which maximize production rate while maintaining acceptable temperature and pressure parameters at the screw tip.
It is yet another object of the present invention to provide a novel extruding screw structure and methods which maximize the rate of extrudate production within an acceptable range of maximum temperature and temperature fluctuations.
It is a further object of the invention to provide novel extruding methods and apparatus which minimize the occurrence pressure pulses during extrudate feeding.
It is still another object of the invention to provide novel extruding methods and apparatus which maximize mixing and circulation of melted and unmelted materials.